The high efficiency of switching power converters such as a flyback converter has led to their virtual universal adaption as the battery charger for mobile devices. Since a flyback converter converts AC household voltage, fault conditions such a short circuit may be potentially dangerous. It is thus conventional for a flyback converter controller to monitor for various fault conditions. Should the controller detect the presence of a fault condition, it stops cycling the power switch and enters a re-startup period. Upon the conclusion of the re-startup period, the controller will again resume normal operation. Should the fault condition reoccur, the controller will again stop cycling the power switch and begin another startup period. The duration of the startup period is thus rather important since if the fault is permanent, it will continually reoccur following each startup period. Should the startup period be too short in the presence of an output short circuit fault condition, the components of the flyback converter may be stressed or damaged by the heat and excessive currents resulting from such a repeated fault.
The duration of the re-startup period is thus essential to minimize power loss and avoid stress to switching power converter components following a fault condition. The re-startup period cannot be too short as discussed above. Conversely, the re-startup period should not be too long or it may exceed user requirements. However, conventional control of the re-startup period suffers from a number of flaws that may be better appreciated with reference to a conventional prior art switching power converter as illustrated in FIG. 1. A controller U1 controls the cycling of a power switch transistor S2 that is in series with a primary winding T1 of the flyback converter's transformer (not illustrated). Depending upon the load demands, controller U1 will switch on power switch transistor S2 through a drive signal applied to its gate. Power switch transistor S2 is in series with a current sense resistor R2 so that controller U1 may measure the primary winding current by sensing the voltage across the current sense resistor. An input voltage V_IN such as produced by rectification of an AC mains voltage drives the primary winding current when the power switch transistor S2 is cycled on.
Controller U1 receives its power supply voltage VCC from a VCC capacitor coupled between a source of a power supply voltage regulator switch transistor S1 and ground. The drain of power supply voltage regulator switch transistor S1 couples through a current limiting resistor R1 to the input voltage rail supplying the input voltage. When controller U1 cycles power supply voltage regulator switch transistor S1 on, the input voltage induces a current through current limiting resistor R1 and power supply voltage regulator switch transistor S1 to charge the VCC capacitor with the power supply voltage VCC. Should controller U1 have to re-start due to a fault condition, controller U1 manages the duration of the re-startup period through cycles of switching off and on power supply voltage regulator switch transistor S1. Each off and on cycle may be designated as a VCC recycling period since charge on the VCC capacitor is “recycled” as the power supply voltage VCC is drained to power controller U1. Some waveforms for the resulting re-startup period are shown in FIG. 2.
At the occurrence of the fault condition, controller U1 enters an initial or first VCC recycling period with the power supply voltage regulator switch transistor S1 off. During an initial portion of the VCC recycling period, controller U1 operates in an active mode such that it draws a relatively large current (Icc_high) from the VCC capacitor. The power supply voltage VCC thus drops relatively rapidly until it hits a threshold value Vcc_low. Controller U1 monitors the power supply voltage VCC to determine whether it has decreased to the Vcc_low threshold voltage, whereupon the controller cycles the power supply voltage regulator switch transistor S1 on. The power supply voltage VCC then begins to increase until it reaches a maximum value Vcc_st at which point controller U1 switches off power supply voltage regulator switch transistor S1. While the power supply voltage regulator switch transistor S1 is on, controller U1 functions in a dormant or sleep mode such that it draws virtually no current from the VCC capacitor.
Controller U1 repeats the VCC recycling period a total of N times to complete the desired re-startup period. Upon the conclusion of the re-startup period, controller U1 resumes normal operation. Re-startup timing control is thus implemented by controlling the number of the VCC recycling periods. In these conventional recycling periods, the duration of each recycling period is dominated by the charging of the VCC capacitor while the power supply voltage increases from Vcc_low to Vcc_st. This VCC charging time is controlled by the resistance of the current limiting resistor R1. This reliance on the current limiting resistor R1 to lengthen the charging time of the VCC capacitor makes the duration of the VCC recycling periods dependent on the input voltage V_IN. For example, when V_IN exceeds its rated voltage, the magnitude of the charging current charging the VCC capacitor increases which results in a shorter VCC recycling period. In turn, this results in a shortened re-startup period. Conversely, a decrease in the input voltage lengthens the duration of the recycling periods and the re-startup period. The prior art dependence on variations in V_IN is thus problematic in that the re-startup timing cannot be accurately predicted.
Accordingly, there is a need in the art for improved re-startup timing control techniques for switching power converters.